SEMINAR ON GLOBAL POSITIONING SYSTEM : For six thousand years, humans have been developing ingenious ways of navigating to remote destination. A fundamental technique developed by both ancient Polynesians and modern navies is the use of the natural stars. With the development of radios, another class of navigation aids was born. These included radio beacons, vhf omni directional radios (VORs), long-range radio navigation (LORAN) and OMEGA. With yet another technology - artificial satellites - more precise, line of sight radio navigation signals became possible. This promise was realized in 1960s, when the US Navy's navigation satellite system known as Transit NOAA opened a new era of navigation technology and capability.
In 1973 a small group of armed forces officers and civilians, sequestered in the Pentagon, had completed a plan that would truly revolutionize navigation. It was based on radio ranging to a constellation of artificial satellites called NAVSTAR's. Instead of angular measurements to natural stars, greater accuracy was anticipated with ranging measurements to the artificial NAVSTAR's. This development was named as GPS (Global Positioning System). This GPS is a satellite based, radio navigation aid designed to eventually provide global, all weather, precise navigation and timing capability to users 24 hours a day. The fundamental navigational technique for GPS is to use one way ranging from the GPS satellites that are also broadcasting their estimated positions. Ranges are measured to four satellites simultaneously in view by matching the incoming signal with a user generated replica signal and measuring the received phase against the users crystal clock. With four satellites and appropriate geometry, four unknowns can be determined, typically, they are latitude, longitude, altitude and a correction to the user clock. If altitude or time is already known, a less number of satellites can be used.

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